Conventionally, some diesel engines are provided with an HC-SCR NOx catalyst (HC-SCR: HydroCarbon-Selective Catalytic Reduction) incorporated in an exhaust pipe for flow of exhaust gas and having an enhanced reaction selectivity capable of selectively reacting NOx (nitrogen oxides) with HC (hydrocarbon) as a reducing agent even in the presence of oxygen. A required amount of fuel is added upstream of the HC-SCR NOx catalyst in a direction of exhaust flow to make reductive reaction, on the HC-SCR NOx catalyst, of HC decomposionally produced from the fuel with NOx in the exhaust to thereby make reduction of NOx emission.
FIG. 1 shows an example of a conventional exhaust emission control device in which reference numeral 1 denotes a diesel engine with a turbocharger 2. Intake air 4 from an air cleaner 3 is passed through an intake air pipe 5 to a compressor 2a of the turbocharger 2. The intake air 4 pressurized in the compressor 2a is passed to an intercooler 6 for cooling and is further introduced from the intercooler 6 to a suction manifold 7 for distribution of the air to respective cylinders 8 of the diesel engine 1 (exemplarily illustrated in FIG. 1 is a case with inline six cylinders). Exhaust gas 9 from the respective cylinders 8 of the diesel engine 1 is passed through an exhaust manifold 10 to a turbine 2b of the turbocharger 2. The exhaust gas having driven the turbine 2b is passed to the exhaust pipe 11.
Incorporated in the exhaust pipe 11 in sequence from an upstream side are a platinum-group-element HC-SCR NOx catalyst 12 having performance of oxidizing NO, a catalyzed particulate filter 13 integrally carrying an oxidation catalyst and a downstream oxidation catalyst 14. Arranged on an entry side of the HC-SCR NOx catalyst 12 is a fuel addition unit 15 for additive supply of fuel to the exhaust gas 9.
The fuel addition unit 15 comprises a fuel tank 16 for storage of the fuel, a fuel pressure pump 17 for pumping of the fuel stored in the fuel tank 16 and a fuel addition nozzle 18 for spray of the fuel pumped by the fuel pressure pump 17 to an entry side of the HC-SCR NOx catalyst 12. The fuel stored in the fuel tank 16 is pumped by the fuel pressure pump 17 to the fuel addition nozzle 18 which sprays the fuel to the exhaust gas 9 flowing through the exhaust pipe 11. In the HC-SCR NOx catalyst 12, HC decomposionally produced from the fuel is reacted with NOx in the exhaust gas 9 to thereby make emission reduction of NOx.
Particulates (particulate matter) in the exhaust gas 9 are mainly constituted by carbonic soot and a SOF (Soluble Organic Fraction) of high-boiling hydrocarbons and contain a trace of sulfate (misty sulfuric acid fraction). The particulates are captured by the catalyzed particulate filter 13 integrally carrying the oxidation catalyst, and HC, CO and the like in the exhaust gas 9 are oxidatively removed by the downstream oxidation catalyst 14.
There exists, for example, Patent Literature 1 showing general state of the art pertinent to the exhaust emission control device as mentioned in the above.